Development of Satellite Positioning Systems (SATPSs), such as the Global Positioning System (GPS) in the United States and the Global Orbiting Navigational System (GLONASS) in the former Soviet Union, has allowed location coordinates of an object on or near the Earth to be determined with improved accuracy. Under ordinary circumstances, these location coordinates can be determined with an inaccuracy of no more than 30 meters. In order to further improve the accuracy provided by SATPS location determination, differential GPS (DGPS), and more generally differential SATPS (DSATPS), has been introduced and used. A DSATPS can provide locations with inaccuracies as low as a few meters, or lower in some instances. Implementation of a DSATPS requires that an SATPS reference station, whose location coordinates are known with high accuracy (to within a fraction of a meter) be provided to receive the normal SATPS signals from an SATPS satellite. The reference station compares its known pseudorange, based on its known location and known satellite and clock biases, with the pseudorange computed using the acceptable SATPS signals received from each visible satellite. The difference, called a pseudorange correction, between the known pseudorange and the computed pseudorange is transmitted for each such SATPS satellite, along with an indicium that identifies that satellite. A mobile SATPS station within 100-1000 kilometers (km) of the reference station receives and uses these pseudorange corrections o correct its own SATPS-determined pseudorange values for each acceptable satellite signal. The pseudorange corrections must be received and processed at the mobile station.
Several problems are presented here. First, this process assumes that the pseudorange corrections, determined at the SATPS reference station, are also valid at the mobile SATPS station, which may be spaced apart from the reference station by as much as 1000 km. This assumption may be unjustified if the local ionosphere and/or the local troposphere is undergoing relatively rapid change with time, or if the multipath signals that contribute to the pseudoranges at the two stations are substantially different.
Second, this process requires that the pseudorange corrections always be transmitted to and used at the mobile SATPS station. In some situations, it may be more convenient to transmit or to download the mobile station pseudorange information to the reference station, or to another supplemental processor station, and to allow the supplemental station to do the processing and subsequent analysis.
Third, the variables actually determined are not the pseudoranges but the locations themselves. A single central station and associated GPS reference station may service a large number of mobile users, each with a different location in the field. The pseudorange corrections for each user varies with the user's actual location in the field. In a tracking application, for example, the GPS-determined location of a mobile user is determined and transmitted to a central station for accumulating a time history of the user's location and for subsequent analysis, using the corrections determined by a GPS reference station at or near the central station. In a mapping application, a sequence of GPS-determined locations are computed and stored in a file in a mobile user's GPS receiver/processor. This file is stored at the central station, to use the corrections determined by a GPS reference station at or near the central station and to develop a corrected set of locations for sites that were earlier mapped by the user.
Although measurements and use of pseudoranges are fundamental to SATPS-assisted determination of location and/or time coordinates, only a few patents disclose procedures that work directly with the pseudorange values. In U.S. Pat. No. 4,578,678, Hurd discloses a GPS receiver that receives a plurality of pseudorange signals, compares these signals with replicas of the expected pseudorange signals, using a correlation technique, and determines the associated time delay, frequency and other variables of interest for these signals to determine receiver location, velocity, clock offset and clock rate.
Keegan discloses a P-code receiver/processor, in U.S. Pat. No. 4,972,431, that analyzes pseudorange and phase for encrypted GPS signals by squaring and filtering the incoming signals. Weaker signals can be analyzed using this technique.
A multi-antenna for GPS signals that determines time biases in the carrier frequencies from time averaging in simultaneous pseudorange measurements is disclosed by Counselman in U.S. Pat. No. 4,809,005. GPS antennas are placed on a seismic survey vessel and on a towed vessel to sense and compensate for false signals received by the seismic vessel antenna(s), and present location of the survey vessel is determined. Similar techniques are disclosed by Counselman, in U.S. Pat. No. 4,894,662, for determining present location using only C/A signals transmitted by the satellites.
Allison, in U.S. Pat. No. 5,148,179, discloses a method for using double differences of pseudorange and carrier phase measurements. The technique uses double differences formed from signals received from four satellites by two different receivers to eliminate certain bias and atmospheric perturbation terms.
A GPS receiver that uses conventional pseudorange and carrier phase measurements to provide a directional indicator, such as a compass, is disclosed in U.S. Pat. No. 5,266,958, issued to Durboraw. A single antenna is moved in a closed path, and differences between predicted and actual carrier phases are used to determine location perturbations, which are then resolved into components parallel and perpendicular to a desired path heading in a given plane.
A networked differential GPS corrections system that provides interpolation of pseudorange corrections (PRCs) between adjacent GPS reference stations is disclosed in U.S. Pat. No. 5,323,322, issued to Mueller et al. Iso-PRC contour specifications are constructed for the regions between the network of reference stations and are transmitted for use by nearby mobile stations.
Kyrtsos et al, in U.S. Pat. No. 5,359,521, disclose positioning of two GPS signal antennas a known distance apart on a vehicle. The pseudorange measurements made at each GPS antenna from GPS signals received from the same satellite are constrained, and the inherent antenna location inaccuracy is assertedly reduced, by accounting for the fixed separation of the two antennas.
U.S. Pat. No. 5,375,059, issued to Kyrtsos et al, U.S. Pat. No. 5,390,125, issued to Sennott et al, and U.S. Pat. No. 5,438,517, issued to Sennott et al, each disclose provision of a first vehicle location, using pseudorange measurements derived from a plurality of GPS satellites and from one or more pseudolites, and simultaneous provision of a second vehicle location derived from an odometer and/or an inertial reference system. The first and second vehicle location are reconciled to provide a third location estimate, using various statistical and/or predictive techniques.
Accuracy of a vehicle location estimate using GPS signals is improved by inclusion of compensation for certain nonlinear errors in the measurements in U.S. Pat. No. 5,390,124, issued to Kyrtsos. Four error coefficients are introduced to model errors inherent in the x-, y- and z-coordinates and in the corresponding pseudorange values.
Kyrtsos et al, in U.S. Pat. No. 5,430,654, disclose provision of a plurality of GPS signal receivers near each other to perform pseudorange measurements from GPS signals received from a given satellite. the pseudorange measurements are then averaged, using appropriate weights, to determine an optimal pseudorange for the general location where the pseudorange measurements are made. Kalman filtering is employed for data extrapolation.
In U.S. Pat. No. 5,430,657, issued to Kyrtsos, the inventor proposes to predict GPS location of a given satellite, using pseudorange and pseudorange rate measurements, and Kalman filter predictions therefrom, made at a sequence of three or more closely spaced times. The inventor asserts that determination of satellite locations by this approach does not rely upon satellite ephemeris data.
Integrity monitoring of the pseudorange and pseudorange rate corrections provided by an SATPS reference station, using an immobile, nearby signal integrity monitoring (SIM) station, is disclosed by Sheynblat in U.S. Pat. No. 5,436,632. If the magnitudes of certain error terms computed by the SIM station are less than threshold values for at least three SATPS satellites, differential SATPS corrections generated by the associated reference station can be used to determine corrected location and velocity coordinates for mobile stations near the associated reference station.
Sheynblat discloses removal of errors from code minus carrier signals due to multipath and/or receiver signal error in U.S. Pat. No. 5,450,448. The code minus carrier signals are modified by one or more statistical processing filters to extract the different signal error components.
U.S. Pat. No. 4,451,964, issued to Babu, discloses provision of pseudorange and carrier phase data from a GPS reference station to a mobile station via a communications link. The mobile station receives these data, applies Kalman filtering and the known reference station and satellite locations to compute pseudorange and carrier phase corrections for itself. Velocity and clock error estimates for the mobile station are determined and used to obtain carrier phase-based estimates of the mobile station location.
Use of pseudorange measurements from GPS satellite signals and from geostationary satellites, plus GPS differential corrections, to provide a location estimate with improved accuracy is disclosed by Dennis in U.S. Pat. No. 5,467,282.
Hatch et al disclose a method for smoothing and reconciling pseudorange (code phase) measurements and carrier phase measurements performed in a GPS signal receiver/processor, in U.S. Pat. No. 5,471,217. Ionospheric and Doppler shift effects are removed from the code phase signals and the results are filtered over extended time intervals.
In U.S. Pat. No. 5,477,458, issued to Loomis, a network of three or more fiducial stations for corrections of carrier phases or of pseudoranges, applicable over a region as large as 3000 km in diameter, is disclosed. A mobile station determines its initially uncorrected location, then determines and applies the carrier phase or pseudorange corrections as provided by the network.
These patents measure pseudoranges, usually at a single station, and apply complex analysis to determine as much as possible from these single-station measurements. Where differential GPS or SATPS pseudorange corrections are sought, a procedure must be found to allow consistent sharing of these corrections and to provide consistent determination of the corresponding location coordinate and clock bias corrections for a reference station and one or more mobile stations that communicate with the reference station.
What is needed is a method and apparatus for converting a mobile user's uncorrected SATPS-determined location in the field to the equivalent uncorrected pseudoranges at the user's location, applying the corrections to the pseudorange values ("innovations") appropriate for the user's location, and determining the user's corrected location coordinates. The pseudorange corrections should be based on the mobile user's location, not on the location of a reference station used for initially determining these corrections. Preferably, this method should be implementable by modest changes made to the existing SATPS location determination software and with no changes in the associated hardware carried by the reference station or by the mobile user. Preferably, this method should allow, but not require, post-processing and should also allow immediate exchange of data for pseudorange corrections, and the amount of data downloaded for processing should be minimized. Preferably, processing of the data should be possible at the reference station, at the mobile station, or at any other supplemental data processor station.